Dart set default value for parameter

BorderRadius.circular() is not a const function so you cannot use it as a default value.

To be able to set the const circular border you can use BorderRadius.all function which is const like below:

class SimpleRoundButton extends StatelessWidget {
  final BorderRadius borderRadius;
  SimpleRoundButton({
    this.borderRadius: const BorderRadius.all(Radius.circular(30.0))
  });

  @override
  Widget build(BuildContext context) {
    return null;
  }
}